1. Field of the Invention
The invention pertains to a process for the production of a pipe fitting, at least certain areas of which consist of thermally weldable material (weld material), which fitting is designed for attachment to, or for the repair of, conduits which also consist of thermally weldable material, wherein several windings of at least one electrical heating wire are first laid in the desired welding area of the future contact surface between the pipe fitting and the conduit, wherein contacts are attached, if desired, to the two ends of the heating wire laid in windings, to which contacts an electrical power source is connected during the later welding of the pipe fitting, and then the position of the windings of the heating wire and possibly the position of the contacts are fixed by welding material which is applied in the injection-molding process. Pipe fittings are used for various purposes to be mounted on a previously installed conduit for media such as gas or water, possibly while the conduit is under the pressure of the medium. Appropriate pipe fittings are also used for repair purposes or to connect sections of pipe. At least certain areas of the pipe fitting and of the conduit consist of thermoplastic resin. In these cases, the pipe fitting has a welding area at its contact surface with the conduit, where several windings of at least one electric heating wire are laid. Contacts which serve to connect the wire to a source of electric power when the pipe fitting is welded on at a later time are usually attached to the ends of the heating wire.
2. Description of the Related Art
In the known process, windings of a heating wire of the type in question can be laid directly in the injection mold used to produce the pipe fitting; this wire is thus integrated into the welding material of the pipe fitting during the injection-molding process (CH 528,697 C). The heating wire is surrounded by a plastic jacket, which hinders the flow of heat to the contact surface between the pipe fitting and the conduit. The windings of the heating wire can be laid in the mold as a monofilar or bifilar spiral or as a meander.
It is also known that a plate-shaped intermediate product can be produced out of the heating wire. This is done by winding the heating wire into a bifilar spiral and by connecting these spiral windings to each other by plastic webs (DE 3,810,845 A1). Finally, it is also known that the heating wire can first be brought into the form of a helix, and that the helix can then be embedded in a star-shaped intermediate product of thermoplastic material (DE 7,121,715 U). When the pipe fitting is injection-molded, this star-shaped intermediate product is then integrated into the area of the contact surface with the conduit. The current flowing through the helix leads to maximum heating in the interior of the helix, however, and this beat is therefore unavailable for the welding process with the conduit. The heat which reaches the contact surface is inadequate. In these known pipe fittings, the strength of the weld to the conduit is unreliable. To achieve satisfactory welding results, it is necessary to use a relatively large amount of electrical energy, which is uneconomical.